2 of the EE new npc's are evil, of a evilness level never seen before, korgan, edvin or viconia are a different thing.
if you run an evil charname it is fine, other way if you have immediately a clue of dorn's goals by the banter when you first meet him is not true in the case of hexxat.
you discover her evil nature only when she is in the party and every good charname should have serious RP problems in simply dismiss her and leaving a powerful vampire in the copper coronet. but she can not be killed, no way your undead hunter pally or good aligned cleric can do his duty.
i found a way to kill her, by the way.
1. as you discover her real nature dismiss her in an area where she can not harm innocent people telling her "wait me here" or let her in the coronet and hope for the best. feel free to use her special bag of holding
2. as you reach the pocket plane in ToB summon her and make her join the party.
3. cast on her GM + doom then turn her to stone, she will turn hostile but being in stone form can not harm you.
4. hit her and she will die with the usual banther where she tells she needs to reach the special coffin.
5. check your inventory, you will have 2 copies of the special bag, really useful to pass items between far away party members as are both doors to the extraplanar space where items are actually stored. but the coffin is now empty even if hexxat should be inside.
6. sleep and she will not appear again, you have killed the npc that can not be killed by developers design!